1. Field of the Invention
This invention relates to radio frequency (RF) circuits and particularly broadband RF circuits such as those used for instance in signal splitters, routers and switching matrices in multichannel systems.
2. Background Information
Typically, multiple RF signals such as television signals offered by cable television (CATV) systems have been distributed using routers made up of many branching elements such as splitters, combiners and mechanical or solid state switches interconnected by a large number of cables. Building large, multi-port routing switchers and matrix switching systems capable of operating in the upper VHF and low microwave frequencies (L-band to 2500 Mhz and beyond) requires a high concentration of splitting, switching and combining elements in limited rack space. To maintain acceptable return loss, insertion loss and isolation performance for each individual path in a "traditional" VHF or microwave switch, each circuit element, switch module and interconnecting coaxial cable has to be individually optimized for all possible (n*M) switching combinations, which is a very time consuming and expensive process, requiring technical expertise, and the outcomes are not predictable due to widely varying physical layouts and cost limitations. This process inevitably results in phasing and gainslope problems, with some paths being deficient at some frequencies over the very broad frequency bands (multi-octaves) of the intended applications.
U.S. Pat. No. 5,481,073 describes a modular broadband switching system suitable for such applications which eliminates most of the cabling by using lumped elements (5-1000 MHz) or distributed elements (above 1000 MHz) and using microstrips to interconnect the multiple branching elements mounted on printed circuit boards (PCB). The routers or matrices are assembled from transversely oriented stacks of parallel sets of such boards (stack & tier configuration). The circuit on an individual board is laid out in a candelabra pattern so that multiple branches between inputs and outputs all contain the same number of branching elements and are all of substantially the same length so that signal loss and phase of each signal in all branches are substantially the same. Initially, the switches used in these circuits were mechanical switches which provide very good isolation, e.g., 90 dB. As the performance requirements placed upon these systems increase, such as by adding data, voice and two-way internet access capability to cable services, the frequency band must be expanded up into the gigahertz range. In addition, efforts have been directed toward increasing the number and switching speed of the channels. This has lead to the use of solid state switches, such as (1*4) element GaAs switches which simultaneously reduce the total count of elements on the circuit boards, while increasing the number of circuit branches and the speed of switching. Unfortunately, these solid state switches do not provide the isolation available from mechanical switches, e.g., typically only 40 dB. This isolation can be improved by adding switches in series with each branch output to increase isolation by as much as about 20 dB at the highest frequency of operation and considerably more at the lower frequencies.
Even with such tandem switching, isolation between branches remains a problem. At least 60 dB of isolation is required between any circuit paths carrying analog TV signals. I have realized that poor isolation is due in part to the virtually universal practice of laying out microstrips on circuit boards in straight lines or arcs of constant radii with adjacent lines or arcs often parallel to each other. While this practice simplifies circuit board design and renders the software used in automated design (CAD) simpler, it leads to coupling between closely spaced lines. In addition, when a signal is split between adjacent parallel lines which radiate the signal in phase, an antenna array may be created. Similarly, an antenna array can be created by two identical lines which diverge about a common axis. Where a common signal is distributed through stacks of identical parallel circuit boards the problem is compounded.
The problem is not limited to broadband multi-channel splitters, couplers, routers and matrices. As the frequency of digital processors increases into the high megahertz and gigahertz ranges the typical linear, parallel patterns of the microstrip lines on mother boards as used on computers and other advanced PCBs also leads to signal isolation difficulties.
There is a need therefore for RF circuits with improved isolation between circuit elements and reduced radiation of signals.